A combustion liner which is used in an aerospace application or in a land based turbine application will have a series of laser drilled holes drilled at an angle to produce a cooling effect during operation. The laser drilled cooling holes are called effusion holes. A typical component will have several thousand effusion holes in order to facilitate the proper cooling pattern. Effusion holes are characteristically drilled at very steep angles (see Figures) to the surface of the component (eg. 170-250). These effusion holes can be laser drilled by three different processes: trepan; percussion drilling; or Laser-on-the-Fly. The trepan method of laser drilling, which pierces the material with a focused beam, then traverses around the hole circumference to produce the hole, is by far the most time consuming. A trepanned laser hole can take from 8 to 12 seconds per hole, dependent on material thickness and angle of entry. The percussion method of laser drilling uses a defocused laser beam to produce the hole by employing a series of pulsed laser shots into the metal until the hole has fully been produced. A percussion drilled hole can take from 1 to 5 seconds per hole, dependent on material thickness and angle of entry. The Laser-on-the-Fly method of laser drilling uses a defocused laser beam, while synchronizing the speed of a rotary device and the pulsing of the laser, to drill a plurality of holes during the rotation cycle one pulse at a time (see U.S. Pat. No. 6,130,405). A hole produced by the Laser-on-the-Fly method can take from 0.3 to 2 seconds per hole, dependent on material thickness and angle of entry.
The typical material that is used to produce a combustor liner is a high temperature stainless steel alloy with a melting point of approximately 2400xc2x0 F. Design engineers, in order to enhance the life expectancy of these components, have added to the component a layer of thermal barrier coating (TBC) having a ceramic top coat. As shown in the Figures, the TBC generally comprises a bond coat 3 to bond the ceramic top coat 2 to the metallic substrate 1. The bond coat can be an MCrAlY bond coat where M is Ni, Co or Fe or a combination of Co and Ni; an aluminide bond coat; or a platinum aluminide bond coat. The ceramic based top coat can be, for example, a zirconia stabilized with yttria. The MCrAlY bond coat can be applied by various processes including plasma spraying, electron beam physical vapor deposition or sputtering, while the ceramic top coat can be applied by various processed including plasma spraying, electron beam physical vapor deposition, sputtering and chemical vapor deposition. The ceramic top coat has a high melting point of, for example, 4500xc2x0 F.
The addition of this thermal barrier coating, while improving component life and engine performance, has created a problem for the laser drilling operation. When laser drilling through the ceramic coating into the base metal as shown in FIG. 4 a large area of recast 4 is created at the intersection of the base metal of the substrate and the thermal barrier coating. This area of recast has been measured up to 0.024 inches thick. The design specifications for combustion liners of several Original Equipment Manufacturers (OEM) have set the maximum allowable recast level at 0.004 inches thick. A recast layer higher than the acceptable limits is detrimental to the life of the component, since a stress crack can ultimately be produced from the recast layer. The pocket of recast is a direct result of the laser""s interaction where the TBC meets the base metal. Since the base metal has a melting point of 2400xc2x0 F., far less than the 4500xc2x0 F. of the ceramic top coat, the molten material has a tendency to create a small pocket at the joining point (see FIG. 3). When employing the percussion or Laser-on-the-Fly drilling method, the pocket is created between the first and second pulses. During the subsequent laser pulses, that are required to fully produce the hole, the molten material is being expelled outward. As the material is being expelled outward, a portion of molten material is being redeposited into the pocket that was created. The solidifying of this material in the pocket forms the xe2x80x9cbubblexe2x80x9d of recast 4 (see FIG. 4).
Many different parameter settings and gas assist combinations were tried to reduce the recast xe2x80x9cbubblexe2x80x9d. The results were similar with all of the combinations that were tested with the recast xe2x80x9cbubblexe2x80x9d clearly present. When employing the trepan method of laser drilling, the recast xe2x80x9cbubblexe2x80x9d was eliminated as the laser beam traversed around the circumference. However, due to the extremely long cycle times that would be required to produce the components with the trepan method, this was not an acceptable solution.